Finding the focus distance for Science Surplus DIY Spectrometer/beam splitter combination (without collimating lens) in Sky Watcher Equinox Pro 80mm Telescope

In my previous post, I described how I have made a device to attach my Science Surplus DIY Spectrometer to my Sky Watcher Equinox Pro 80mm Telescope using a beam splitter:

Making a device to connect the fiber optic cable from the DIY Spectrometer to a telescope & adding in some way to guide it in use – Part 1, Attempt 1

In my follow up posts, I described my initial experiences of using this combination:

Spectral response of the DIY Spectrometer & why I can’t detect red

Spectra of coloured LED torches & white LED & Compact Fluorescent Bulb taken using DIY Spectrometer, ATM beam-splitter telescope-optical fiber connector, Sky Watcher Equinox Pro 80mm

Calibrating DIY Spectrometer+beam splitter+Sky Watcher Equinox Pro 80mm combination 26/8/2018 using compact fluorescent light bulb

Spectra of lights in lounge using DIY Spectrometer and Sky Watcher Equinox Pro 80mm and beam splitter 25/8/2018

Today, I tried to work out the best focal distance on the Sky Watcher Equinox Pro 80mm Telescope for the beam splitter so that I optimised the signal reaching Spectrum Studio software.

Note that these experiments were done without a collimating lens before the optical fibre.

Sky Watcher 80mm Pro Telescope with beam splitter/optical fibre cable connected to computer running Spectrum Studio software:

This experiment was done using a compact fluorescent bulb at other end of room – in order to bring the intensity of the light down sufficiently to avoid plateauing out on Spectrum Studio software, I had to cover the light with a double layer of cloth:

The following photo shows the back of the beam splitter with the optical fibre cable removed – the image of the light is projected on to the white sheet of paper. I was able to move the paper until the light was maximally focused. Then changing focus on the tube until this focus point was just inside the back of the beam splitter meant I was close to maximal intensity on Spectrum Studio when the fibre optic cable was reconnected.

I found that I could change focus on the Equinox until maximal intensity was found on Spectrum Studio software. The following three photos show the intensity on Spectrum Studio with the focus tube racked in as far as it will go, at maximal intensity on Spectrum Studio and with tube racked out.

Focus tube racked in – intensity 23,000:

Focus tube at maximal intensity on Spectrum Studio – intensity 42,000:

Focus tube racked out – intensity 37,000 (starting to drop again):

The drawtube is at 46mm when Spectrum Studio shows maximal intensity:

With the focus tube set at 46mm, I then determined where the eyepiece needs to be placed in the guiding port (pointing towards us – illuminated eyepiece in it) so that it is also in focus with the focus tube set at 46mm.


From this I determined that I need to purchase a 1.25″ extension tube 17.4-12.9cm = 4.5cm (45mm) in length.

Spectrum taken with Sky Watcher Pro 80mm Telescope/beam splitter/DIY Spectrometer:

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